Modern pulse time-of-flight distance measuring apparatus such as laser range finders or laser scanners work at a high pulse power over large distances and/or at a high pulse repetition rate to quickly create a large number of measurement points of the environment. Both cases may result in the situation that the next pulse is already transmitted before the reflection of the last pulse was received, so that the received pulses cannot be clearly mapped anymore to their respective transmitted pulse. This is known as the “Multiple Time Around” (MTA) or “Multiple Pulses in the Air” problem. In this context, the maximum size dmax of the range of unambiguously measurable distances, the so-called MTA zone, follows from the pulse repetition rate PRR and the speed of light c as:dmax=c/(2·PRR).
Laser scanners of modern design for instance offer pulse repetition rates of up to 400 kHz, which corresponds to a MTA zone size dmax of about 375 m. If this measuring distance is exceeded, the result of the measurement usually cannot be interpreted correctly, as the transmitted and received pulses cannot be unambiguously mapped.
FIGS. 1 and 2 show this situation in detail. An airborne laser scanner 1 emits a pulsed laser measuring beam 2 which scans an environment U having single targets (scan points) U1, U2, . . . , e.g. fan-like line by line. Time-of-flight measurements at the single transmitted pulses S2, S2, . . . which are returned as received pulses E1, E2, . . . following the external reflection, serve to determine the distances D1, D2, . . . to the individual targets U1, U2, . . . .
FIGS. 1a and 2a show an exemplary situation in the measurement of targets U1, U2 which are located in the first MTA zone Z nearest to the laser scanner 1: The received pulse E1 belonging to the transmitted pulse S1 is returned before the next transmitted pulse S2 is transmitted in the time interval τ=1/PRR, etc.
FIGS. 1b and 2b show an exemplary situation where targets U3′, U4′ are located in the second MTA zone Z′: In this case, the received pulse E3 belonging to the transmitted pulse S3 is only received after the second transmitted pulse S2 was emitted. In order to determine the correct distance D3′ of the external target U3′ in the zone Z′, it is necessary to correctly map the received pulse E3 to the transmitted pulse S3; if the received pulse E3 is wrongly mapped to the immediately preceding transmitted pulse S4, this will result in a wrong target distance D3 in the wrong MTA zone Z instead of the correct target distance D3′ in the correct MTA zone Z′.
In order to correctly map the received pulses to the transmitted pulses and thus to overcome the MTA zone boundaries for achieving unambiguous distance measuring results, different methods are known in the art. A first option is to make sure in planning the measurement that all targets to be expected are located in one and the same MTA zone so that the correct mapping can be made. This method is naturally only applicable to special measurement tasks and is not suitable e.g. for highly mobile or large scale measurement or scanning tasks, e.g. the airborne scanning of mountains or the terrestrial vehicle-based scanning.
Another group of methods is based on making the individual transmitted pulses distinguishable from one another by variation of their polarization, amplitude or wavelength so that the received pulses can be mapped accordingly. However, these methods are either only suitable for just a few number of “pulses in the air” or require elaborately coded pulses, which both limits the pulse repetition rate and range of measurable distances and prolongs the time of measurement.